1. Field of the Invention
The present invention relates to a high-pressure fuel injection tube having a diameter as small as about 30 mm or less, which is arranged as a fuel conduit in the vicinity of a diesel engine, especially a marine engine.
2. Description of the Prior Art
In order to completely prevent the dispersion and leakage of a fuel to the outside of a marine engine and fire on a ship on the sea, generally speaking, a high-pressure fuel injection tube of the above-specified type is constructed such that a joint cylinder 13 fitted on separate outer tubes 12 and 12' fitted axially movably on the outer circumference of a tube body 11 while leaving a gap 14 between their opposed end faces has its two end portions welded at W and W' to the inner end portions of the outer tubes 12 and 12' to make a double wall structure.
In this structure of the prior art, however, with the output tubes 12 and 12' being fitted in position on the tube body 11, the joint cylinder 13 is fitted to have its two end portions soldered or welded at W and W' to the outer tubes 12 and 12' This welding step is troublesome and invites the deterioration of the mechanical strength due to the heating treatment of the tube body 11 in the vicinity of the selded portions and the deterioration of corrosion-resistance due to th thermal damage of the anti-corrosive surface films of the outer tubes 12 and 12'. Specifically, the welding step is required at the two portions after the assembly has been ended to a substantial product state. At the same time, the film plated with Zn or the like is damaged by the heat treatment and has to be repaired by a painting step or the like. On the other hand, the tube body 11 is prepared by elongating and heating a mother tube of steel and/or stainless steel repeatedly to give it several characteristics such as the plactic hardness. As a result of the aforementioned welding treatment W and W', the steel tube has a tendency to become fragile or soft due to the course crystal structure and the annealed structure. The stainless steel tube is degraded due to the Cr-C deposition. By the deterioration in the mechanical strength and the vibration from the engine, the tube used in the engine has its wall cracked after a long time use to induce the dispersion and leakage of the fuel in the spray form to the outside. Thus safer counter-measures are desired at present.